1. Field of the Invention
The present invention relates to an organic electroluminescence display having a display array or a regular arrangement of a plurality of light emitting parts which consist of organic electroluminescence devices each having a light-emitting layer made of a thin film of organic electroluminescence material utilizing electroluminescence (hereinafter, also referred to as EL) of organic compound material in which luminescence is produced by electron or hole injection.
2. Description of the Related Art
An organic EL display, comprising a plurality of organic EL devices arranged in a matrix, is receiving attention as a display that is low in power consumption, high in display quality, and capable of low-profile configuration.
The organic EL device is known as a self-luminescence device in which at least one organic material layer and a metal electrode are laminated on a transparent substrate. An example of the transparent substrate is a glass plate provided with a transparent electrode of indium-tin oxide, i.e., so-called ITO. Examples of the organic material layer include an organic electron-transporting layer, an organic light-emitting layer, and an organic hole-transporting layer. Positive and negative voltages are respectively applied to the transparent electrode and the metal electrode, i.e., anode and cathode, for charge accumulation. When a barrier voltage i.e., a luminescence threshold voltage inherent to the device is exceeded, a current starts to flow and there occurs luminescence with certain intensity approximately proportional to this direct current.
Among display panels that use organic EL devices are a simple matrix type display panel in which the organic EL devices are simply arranged in a matrix, and an active matrix type display panel in which the organic EL devices arranged in a matrix are provided with an additional transistor-based driving element each. When compared to the simple matrix type display panel, the active matrix type display panel has such advantages as low power consumption and low interpixel crosstalk. The active matrix type display is suited to large-screen display and high-definition display in particular.
The display of active matrix drive system exercises switching with respect to each of the light emitting parts by using thin film transistors (TFTs) which are made of polysilicon, for example. Consequently, a current is supplied pixel by pixel to cause luminescence of the organic EL devices. MOS-FETs (Metal Oxide Semiconductor Field Effect Transistors) are used as the TFTs.
A MOS-FET has two reverse conductive regions of polysilicon which are formed on a glass substrate, for example. An oxide (SiO2) thin film and a metal gate electrode are formed in succession on the surface of the substrate, between the reverse conductive regions, so that the conductivity is controlled by the field applied from the metal gate electrode. Since the display substrate must be a polysilicon substrate or the like which requires a high temperature treatment, and films of inorganic material such as Si must be formed thereon, the fabrication involves high temperature process.
Displays having large-screen display panels are in high demand. There has been developed a low-temperature polysilicon substrate thereof. Despite the name “low-temperature,” however, the substrate requires a heat treatment on the order of 500° C. at the time of fabrication. Anyway, incorporating inorganic material TFTs into the large-screen display panel of an organic EL display of active matrix drive system inevitably pushes up the cost of the display.
Then, there has been proposed an organic TFT that has an organic semiconductor film of organic material formed between a pair of opposite electrodes. It is considered that this organic TFT can be used to drive organic EL devices.
Nevertheless, there has not been proposed any concrete organic TFT structure. Moreover, an organic semiconductor material and an organic material layer, structurally indispensable to the organic EL devices to be driven by the organic TFT, are both extremely poor in heat resistance, solvent resistance, moisture resistance, and so on. It has thus been difficult to achieve a practical organic EL display panel.